136 REINFORCED CONCRETE CONSTRUCTION 



figuring stresses, the beam and slab must be well tied together 

 by the steel reinforcement. 



With cross-beam and girder construction, the span of the slab 

 will usually range from 4 to 6 ft. In such short spans, the slabs 

 become rigidly supported if they are well bonded to the beams, 

 by reason of the action of the adjoining floor-panels. Even 

 where the slabs are simply supported on the tops of steel beams, 

 the adjoining slabs prevent any lateral motion and render the 

 slab partially continuous. With supports in monolithic con- 

 struction, therefore, the strengthening effect is especially great 

 and reinforcement against negative moment is hardly necessary. 

 For such short spans, a reinforcement of rods near the bottom 

 will be effective. They should be laid with lapped and broken 

 joints to give continuity and to prevent the localization of 

 contraction cracks. (Fig. 67.) 



FIG. 67. 



In the case of spans longer than 5 or 6 ft. it becomes necessary 

 to reinforce against negative moment. This may be done in the 

 same manner as already suggested by bending up a part of 

 the rods and extending the bent ends beyond the beam. The 

 bend in the bars should be near the 1/4 points in the span, and 

 usually at an angle of 30 degrees with the horizontal. Too 

 sharp an angle may tend to crack the slab. Fig. 68 illustrates 

 two arrangements of rods. For clearness the rods are shown one 

 above the other but, with the exception of the bends, they are 

 actually in the same horizontal plane at the different points and 

 are spaced alternately. Bars may extend over several spans if 

 the spans are short, but in such cases they should be arranged to 

 break joints so as to accomplish the same results as shown in 

 Fig. 68. 



Fig. 69 is another arrangement to accomplish the same object, 

 using separate straight rods. Either arrangement in Fig. 68 is 

 preferable to that shown in Fig. 69 for very heavy loads, in order 

 to provide against shearing failures. Shearing failures are not 

 usually important in slabs, but in special cases of heavy loading 

 the same care should be used as in the design of large beams. 



